Bismuth selenide is a gray powder that is a compound of bismuth

Bismuth

Bismuth is a chemical element with symbol Bi and atomic number 83. Bismuth, a trivalent poor metal, chemically resembles arsenic and antimony. Elemental bismuth may occur naturally uncombined, although its sulfide and oxide form important commercial ores. The free element is 86% as dense as lead...

 and selenium

Selenium

Selenium is a chemical element with atomic number 34, chemical symbol Se, and an atomic mass of 78.96. It is a nonmetal, whose properties are intermediate between those of adjacent chalcogen elements sulfur and tellurium...

 also known as bismuth(III) selenide. It is a semiconductor

Semiconductor

A semiconductor is a material with electrical conductivity due to electron flow intermediate in magnitude between that of a conductor and an insulator. This means a conductivity roughly in the range of 103 to 10−8 siemens per centimeter...

 and a thermoelectric material. While perfect stoichiometric bismuth selenide should be a semiconductor (with a gap of 0.3 eV) naturally occurring selenium vacancies act as electron donors

Doping (semiconductor)

In semiconductor production, doping intentionally introduces impurities into an extremely pure semiconductor for the purpose of modulating its electrical properties. The impurities are dependent upon the type of semiconductor. Lightly and moderately doped semiconductors are referred to as extrinsic...

 and it often acts as a semimetal. Topologically protected surface states

Topological insulator

A topological insulator is a material that behaves as an insulator in its interior or bulk while permitting the movement of charges on its surface....

have been observed in Bismuth selenide which is the subject of ongoing scientific research.